Nanoscale magnetic configurations of supported Fe nanoparticle assemblies studied by scanning electron microscopy with spin analysis

Microscopic magnetic behavior of supported nanoparticles is strongly correlated with their functionalities, especially in data storage and biological applications, but still needs to be clarified. We studied nanoscale magnetic configurations of Fe nanoparticle assemblies using scanning electron microscopy with polarization analysis. The flux closure domain configurations and the reduced magnetic correlation length $\left(\sim 250\text{nm}\right)$, relative to the conventional thin films, are determined. Quantitative analysis indicates the magnetic interaction energy to be 80–99 meV, close to the magnetic dipolar coupling energy. These direct observations evidence the aforereported simulations and will be valuable for fabricating magnetic nanoparticle assemblies with the desired magnetic properties.

Figure 1

(Color online) STM images of (a) 9 and (b) 13 ML Fe nanoparticle assemblies on ${\text{Al}}_2{\text{O}}_3/\text{NiAl}\left(100\right)$ with the indicated line profiles shown in the bottom. (Image size: $300\times 300\text{nm}$)

Figure 2

(Color online) In-plane MOKE hysteresis loops of (a) 9 and (b) 13 ML Fe nanoparticle assemblies on ${\text{Al}}_2{\text{O}}_3/\text{NiAl}\left(100\right)$. $\theta =0°$ and $45°$ are measured with the magnetic field along [010] and [011] directions, respectively.

Figure 3

(Color online) SEMPA images of (a) 9 ML Fe nanoparticles, (b) 13 ML Fe nanoparticles, and (c) 9 ML Fe film with the histogram of the magnetization directions. The arrows indicate the magnetization directions. $\text{Angle}=0°$ in the histogram is parallel to [010]. The white circles in (b) mark the flux closure magnetic domain structures. The inset exhibits the rotation of magnetization angle along the indicated line crossing the center of a flux closure domain. (d) The correlation functions of 9 and 13 ML Fe nanoparticles, indicating the magnetic correlation length to be $\sim 250\text{nm}$.

Figure 4

(Color online) (a) SEMPA and (b) STM images of the same scale for 13 ML Fe nanoparticles are exhibited, in order to reveal the idea of that each pixel represents the magnetization in a $10\times 10{\text{nm}}^2$ area, close to the limit of the particle size. (c) Histogram of angle differences between nearby pixels in SEMPA images of 9 ML Fe nanoparticles, 13 ML nanoparticles, and 9 ML films (as shown in Fig. 3.) The inset shows the magnified histogram of 9 and 13 ML Fe nanoparticles with fitting curves according to Eq. (1).